Photo-SABRE: Nuclear Spin Hyperpolarization of cis-trans Photoswitchable Molecules by Parahydrogen

This work reports the 23000- and 9200-fold enhancement of the N-15 nuclear spin polarization of cis and trans isomers of azobenzene, correspondingly, as compared to the thermal NMR signals at 9.4 T by exploiting signal amplification by reversible exchange (SABRE) and parahydrogen molecules at 400 nT simultaneously with light irradiation. Only cis-azobenzene gains substantial hyperpolarization of its N-15 spins directly from parahydrogen in SABRE through coherent polarization transfer. Consequently cis-trans photoisomerization at ultralow magnetic field preserves the SABRE-derived nuclear hyperpolarization of cis-azobenzene resulting in hyperpolarization of trans-azobenzene as well, despite its direct coordination with the SABRE Ir-complex being sterically hindered. Moreover, the proposed approach, which we called photo-SABRE, allows to hyperpolarize the long-lived spin order of trans-isomer N-15 spins with a lifetime of about 25 minutes, which greatly exceeds their relaxation times at high (10 seconds) and low (200 seconds) magnetic field. Since this spin order in N-15(2)-trans-azobenzene is collectively formed by nuclei of different kinds, it can be detected by both N-15 or H-1 NMR spectroscopy.

Automated summary

This study demonstrates a significant enhancement of nuclear spin polarization of cis and trans isomers of azobenzene by utilizing reversible exchange and simultaneous interaction with parahydrogen molecules and light irradiation. The approach of coherent polarization transfer allows for direct hyperpolarization of cis-azobenzene from parahydrogen in SABRE. Additionally, the proposed method, referred to as photo-SABRE, enables hyperpolarization of the long-lived spin order of trans-isomer N-15 spins.